Traditional fluorescent anti‐counterfeiting labels can be easily cloned due to the lack of sufficient security. Therefore, multilevel anti‐counterfeiting techniques with higher security need to be constantly explored. This study reports a multilayer composite structure containing Mn doped CsPbCl3 (Mn:CsPbCl3) perovskite quantum dots (PQDs) layer and CsPbBr3 PQDs layer. The encrypted information is patterned on the Mn:CsPbCl3 PQDs layer with weak orange fluorescence, which is concealed by the brighter green fluorescence of CsPbBr3 PQDs at the upper layer. Based on the well‐separated photoluminescence (PL) peak positions, inverse PL temperature‐dependences and distinct PL lifetimes of these two PQDs, the encrypted information can be read out by adding cut‐off filter, heating and using ultrafast camera (UCA)/pulsed excitation (PE). This multilevel anti‐counterfeiting label significantly increases the difficulties of imitation meanwhile no expensive equipment is required for the verification of authenticity. Moreover, two bits of binary numbers that can be represented by dual emissions of the Mn:CsPbCl3 PQDs by changing the doping content, which significantly increases the information storage capacity, are also demonstrated.

No abstract

Despite the demonstrated high efficiency of solar cells and light-emitting devices based on two-dimensional (2D) perovskites, intrinsic stability of the 2D perovskites is yet far from satisfactory. In this work, we find the 2D (BA)2PbI4 perovskite crystals rapidly degrade in the ambient conditions and the photoluminescence (PL) nearly completely quenches in 6 days. Moreover, the PL shoulder band due to defects and absorption band of PbI2 gradually rise during degradation, suggesting the precipitation of PbI2. Besides, rod structures are observed in the degraded crystals, which are attributed to the formation of one-dimensional (1D) (BA)3PbI5 perovskites. And the degradation can be largely retarded by decreasing the humidity during storage. Therefore, a chemical reaction for the degradation of (BA)2PbI4 is proposed, revealing the interactions between water molecules and undercoordinated defects are very critical for understanding the degradation. Enlightened by these findings, dimethyl itaconate (DI) treatment is developed to passivate the defects and block the intrusion of moisture to improve the stability of the (BA)2PbI4. After storage in the ambient environment for 16 days, the DI treated (BA)2PbI4 only shows a slight surface degradation without formation of any nanorod-like structures, and the PL intensity retains about 70%. Therefore, our systematic study provides a comprehensive understanding on the degradation dynamics of 2D perovskites, which will promote future development of intrinsically stable 2D perovskites.

Multicolor fluorescence of mixed halide perovskites enormously enables their applications in photonics and optoelectronics. However, it remains an arduous task to obtain multicolor emissions from perovskites containing single halogen to avoid phase segregation. Herein, a fluorescent composite containing Eu-based metal-organic frameworks (MOFs), 0D Cs 4 PbBr 6 , and 3D CsPbBr 3 is synthesized. Under excitations at 365 nm and 254 nm, the pristine composite emits blue (B) and red (R) fluorescence, which are ascribed to radiative defects within Cs 4 PbBr 6 and 5 D 0 → 7 F J transitions of Eu 3+ , respectively. Interestingly, after light soaking in the ambient environment, the blue fluorescence gradually converts into green (G) emission due to the defect repairing and 0D-3D phase conversion. This permanent and unique photochromic effect enables anticounterfeiting and microsteganography with increased security through a micropatterning technique. Moreover, the RGB luminescence is highly stable after encapsulation by a transparent polymer layer. Thus, trichromatic light-emitting modules are fabricated by using the fluorescent composites as color-converting layers, which almost fully cover the standard color gamut. Therefore, this work innovates a strategy for construction of tunable multicolor luminescence by manipulating the radiative defects and structural dimensionality.

No abstract